Parametric amplifier modulation expander



Nov. 8, 1960 R. ADLER PARAMETRIC AMPLIFIER MODULATION EXPANDER Filed May1, 1959 ELECTRON MODULATION EXPAN DER SIGNAL GENERATOR lNVE/VTOR 20522-2oZaZez" United States Patent PARAMETRIC AMPLIFIER MGDULATION EXPANDERRobert Adler, Northfield, Ill., assignor to Zenith Radio Corporation, acorporation of Delaware Filed May 1, 1959, Ser. No. 810,302

4 Claims. (Cl. 3305) The present invention is directed generally toparametric amplifiers of the fast wave transverse field type andconcerns in particular the construction of a modulation expander forsuch an amplifier.

Parametric amplifiers may employ longitudinal fields in place oftransverse fields as explained in a copending application of RobertAdler, Serial No. 738,546, filed May 28, 1958 and assigned to theassignee of the present invention. The material considered herein,however, has particular application to the transverse mode device andfurther consideration of the parametric amplifier will be restrictedthereto.

As explained in the Adler application, a transversemode parametricamplifier comprises an electron gun for projecting a beam of electronsalong a predetermined path to a collector or final anode. A focusingfield which may be established by means of a solenoid encompassing aportion of that path may establish electron resonance in the beam and asignal may be transferred to the beam by means of a coupling devicewhich is excited from the signal source and which is arranged along thebeam path for interaction with the stream. This coupler may be a lumpedor a distributed device and may be employed for imparting signal energyto the beam while at the same time extracting fast transverse noisecomponents carried by the beam into the field of the coupler or inputsignal modulator as it is sometimes called. The interaction of themodulator and stream modifies the electron motion in accordance with thesignal energy, causing the electrons to assume a helical or orbital pathof travel under the influence of the signal.

Amplification of the signal carried by the stream is accomplished bymeans of a structure referred to as an electron modulation expanderwhich expands the electron motion of the stream. One especiallyattractive form of modulation expander is the quadrupole structuredescribed and claimed in a copending application of Glen Wade, SerialNo. 747,764, filed July 10, 1958 and assigned to the assignee of thepresent invention.

The quadrupole structure is an arrangement of four electrodes excited ina particular relation by a pumping signal to establish an inhomogeneouselectric field alternating at twice the electron resonance frequency.The field forces acting upon the electrons of the stream as it traversesthe modulation expander occasion a net exponential growth of electronmotion and thus gain of the signal carried by the stream.

Interposed between this modulation expander and the final collector isan output coupler or demodulator which, in the. usual construction, isidentical to the input modulator. It extracts the amplified signalenergy from the beam for application to a load.

The described structure has been found to operate very satisfactorily inaccomplishing parametric amplification of applied signals. It will berecognized that the quadrupole modulation expander is a lumped asdistinguished from a distributed type of circuit structure having a highQ. It functions optimally at a particular pumping frequency.

As an incident to the modulation expansion, a signal component referredto as an idler wave is established on the stream as a modulationproduct. This phenomenon is explained in both the Adler and Wadeapplications and is related to the pumping and signal frequencies inaccordance with the following equation:

where n is the signal frequency, 01 is the idler frequency and 00 is thepumping frequency.

It is apparent from Equation 1 that a modulation expander capable ofeflicient operation over a band of pumping frequencies permitsflexibility in the selection of operating frequencies to effectamplifiction at a particular signal frequency. More particularly, itaffords a choice of pumping frequency by means of which the idlercomponent may be adjusted as to frequency and this may be highlydesirable in particular installations. For example, if in a giveninstallation it is found that an idler frequency of a particular valueis undesirable because of the environment, the pumping frequency may bechanged to displace the idler frequency-wise to avoid this objection.The arrangement to be described herein facilitates achieving thisflexibility in the modulation expander of a parametric amplifier.

Accordingly, it is an object of the invention to provide a novelmodulation expander for a fast wave transverse-mode parametricamplifier.

It is a particular object of the invention to provide a novel modulationexpander for a parametric amplifier characterized by the fact that itmay accommodate pumping frequencies within a selected frequency band.

A fast wave transverse-mode parametric amplifier employs an electronbeam modulated with signal energy and projected along a predeter'rnnedpath to a parametric modulation expander. Such an expander, inaccordance with the instant invention, functions over a band of pumpingfrequencies centered about a given frequency. The expander comprises atransmission line wrapped about a portion of the beam path in accordancewith a generally helical pattern so dimensioned that each turn of thehelical pattern has an effective electrical length equal to twice thewave length at the aforesaid given or center frequency of the band ofpumping frequencies.

The features of the present invention, together with further advantagesand benefits thereof, will be more clearly understood from the followingdescription of particular embodiments thereof taken in conjunction withthe annexed drawing in the several figures of which like components aredesignated by similar reference characters and in which:

Figure 1 is a schematic diagram of one form of fast A describing theoperation of the subject expander and its relation to the quadrupoletype of modulation expander described and claimed in theabove-identified Wade application.

Referring now more particularly to Figure l, the parametric amplifierthere represented comprises a source or electron gun 10 for developingand projecting a stream or electron beam along reference path 11, 11.The beam source may be entirely conventional and preferably includes theusual cathode together with suitable focusing and acceleratingelectrodes for developing a Well-defined beam of electrons. Forconvenience of illusl- Patented Nov. 1960 tration, this source has beenrepresented merely by the usual symbol for an indirectly heated cathode.An electron beam collector 12 is disposed at the end of the path remotefrom source and usually takes the form of an anode biased at a positivepotential with respect to the cathode as indicated by potential source+8.

The amplifier has means for creating in the beam path a field forestablishing electron resonance in the beam traversing that path. Whileelectron resonance may be established through the agency of a magneticor an electrostatic field, the arrangement in question is indicated as asolenoid 13 surrounding the beam path to establish lines of magneticflux parallel thereto and of a strength establishing a selectedcyclotron frequency for electron motion. The focusing field of thesolenoid is indicated symbolically by arrow H Also spaced along beampath 11, there are means for modulating the electron beam in response toan applied signal frequency. This modulating means is an electroncoupler capable of imparting energy to the beam in response to signalenergy received from a source 16. Different forms of coupling structuresmay be employed in parametric ampifiers of the transverse-mode type.They may, by way of illustration, be resonant cavities, transmissionlines, or deflection plates spaced alongside the beam for interactiontherewith. As illustrated, modulator 15 includes a pair of deflectorplates 17, 18 located on opposite sides of the beam path. For couplingsignal source 16 to the modulator, a transmission line 19 having one end20 short-circuited is coupled at its opposite end to deflectors 17, 18.A transmission link 21 is tapped as indicated at 22 onto transmissionline 19 in a position adjusted to match the impedance of source 16 tothat presented by deflectors 17, 18. Transmission line 19 has aneflective electrical length of one quarter wave length at the frequencyof the signal from source 16.

Amplification is obtained in the parametric type of amplifier by meansof a modulation expander for expanding the signal modulation of thebeam. A modulation expander 25 is shown in block form in Figure 1 andits structure will be described in detail hereinafter. Sufiice it forthe moment to say that the modulation expander is a structure withinwhich is established a restoring force or suspension for the electronsof the beam. This may conveniently be accomplished by the creation of ahomogeneous magnetic focusing field represented symbolically by thearrow H which establishes electron or cyclotron resonance of the samevalue as that of input coupler 15. This focusing field may be developedby a separate solenoid included within the modulation expander but it ismore convenient structurally to make use of a single elongated solenoidwhich encompasses not only the input modulator 15 but also modulationexpander 25 and an output coupler further along the beam path than themodulation expander and provided for a purpose to be consideredpresently.

Energy from which signal amplification is eventually derived is suppliedby means of a driving or pumping signal generator included in expander25 to produce an alternating inhomogeneous field having a frequency ofapproximately twice the cyclotron frequency. The time varyinginhomogeneous field varies the stifiness of the electron suspensioncreated by field H periodically so as to impart energy to the electronmotion.

As previously indicated, still further along beam path 11, betweenmodulation expander 25 and anode 12, is an output coupler or demodulator30 serving to extract the amplified signal from the beam for applicationto a load 31. The output coupler is identical in structure to inputcoupler 15 and its component parts bear identical reference characters.

As thus far described the system of Figure l is a parametric amplifierof the general type described in both the Adler and Wade applications.If the modulation expander 25 is a quadrupole type of structure, thearrangement is then identical to that of the Wade application andfurther identical to that described in an article entitled ParametricAmplification of the Fast Electron Wave by Robert Adler, published inProceedings of the IRE, volume 46, No. 6, June, 1958 and a companionpaper entitled A Low Noise Electron Beam Parametric Amplifier by RobertAdler, George Hrbek and Glen Wade published in the same publication,Volume 46, No. 10, under date of October, 1958.

In operation, an electron beam issued from source 10 enters the field ofthe input coupler or modulator 15 wherein it is modulated with theapplied signal from source 16. It may be assumed that the signalfrequency is essentially the same as the electron resonant frequencyestablished by focusing field H and interaction of the beam and inputmodulator results in transferring signal energy from deflection plates17, 18 to the beam. At the same time, fast wave electron beam noise orother fast wave signal components carried by the beam are surrendered tothe input coupler, purging the beam of undesired signal components. Thebeam, as it leaves modulator 15, is characterized by electrons moving inan orbital or helical path such that the electron motion represents theapplied signal. The time varying inhomogeneous field of the modulationexpander 25 created under the influence of the pumping signal sourceexpands the electron motion and effects amplification of the signalcarried by the beam. The beam then enters output coupler 30 and theamplified signal is extracted from the beam and delivered to load 31. t

More detailed consideration will now be given to the structure ofmodulation expander 25 with particular reference to Figure 2. As thererepresented, the expander is a transmission line 40 which is wrappedabout a portion of beam path 11 intermediate the input and outputcouplers. Preferably, the expander is in the form of a double helix inthat the transmission line is itself a helix and the line is wrapped inaccordance with a generally helical pattern of such dimension that theeffective electrical length of each turn of the helical pattern is equalto twice the wave length of the center frequency of a desired band ofpumping frequencies. The line is constructed to be substantiallydispersionless over this band, that is to say, the line is constructedto exhibit a substantially constant velocity of wave propagation overthe band. This may be achieved with a distributed transmission linestructure over a band of useful width.

Of course, the structure including the electron beam source 10, inputand output modulators 15 and 30, helix 40 of modulation expander 25 andcollector or anode 12 is housed within a tube envelope as represented bythe broken-line construction and as described in both theabove-identified Adler and Wade applications. Consequently, anyconventional supporting device may be adapted to the end thattransmission line 40 is mechanically secured within the envelope inproper space relation to the beam path. For example, it is entirelyappropriate to support the outside of helix 40 between three or fourinsulating supporting rods arranged parallel to the beam path 11. Thecoil 13 indicates the magnetic focusing structure for establishing thedesired cyclotron frequency but, as pointed out earlier, in practicalconstructions of the tube a single elongated solenoid is em ployed toincorporate modulators 15 and 30 and the modulation expander.

One end of transmission line 40, the end adjacent input modulator 15, iscoupled to a pump signal generator 41. This may be a conventionaloscillator tunable over a desired band of pumping signal frequencies.The opposite end of the transmission line is terminated in a resistor 42selected to provide a matching termination for the line to avoidundesirable reflections.

In order to avoid undesirable electrostatic lens effects along the pathof travel of the electron stream, suitable direct current potentials maybe applied from a source (not shown) to the defiection plates ofmodulators 15 and 30 and to helix 40 of the modulation expander.

The operation of the modulation expander of Figure 2 will be explainedwith reference to Figures 3a-3c and it will be assumed initially thatpump signal generator 41 has been adjusted in frequency to the center ofits operating frequency range. It will be further assumed that thisfrequency is twice the cyclotron frequency. For the assumed conditions,each turn of the helix defined by transmission line 44) has an effectiveelectrical length that is twice the operating wave length of the pumpgenerator. The potential distribution of Figure 3a is established atsome instant on the turns of the helix 40. The top and bottom portionsof each turn are at a peak positive potential whereas the left and righthand portions of each turn, midway between the top and bottom,experience peak negative values. Since this potential distribution istrue of every turn of the helix for the assumed conditions, the expanderis fully analogous to the quadrupole structure described in the Wadeapplication and in the Adler et al paper in the October, 1958Proceedings of the IRE.

The quadrupole structure is represented in Figure 3b and comprises fourelectrodes 50, 51, 52 and 53 symmetrically disposed circumferentiallyaround the beam path 11. Each electrode has the shape of an equilateralhyperbola and the electrodes are disposed with their intermediateportions facing the beam path and their terminal portions projectingoutwardly therefrom with each terminal portionspaced generally parallelfrom the adjacent terminal portions of the neighboring electrodes.Oppositely disposed electrodes 50, 52 are coupled to one terminal ofpump generator 41 and the other pair of oppositely disposed electrodesare coupled to the opposite terminal of the pump. At the instant inquestion, electrodes 51 and 53 are positive while electrodes 50, 52 arenegative. Accordingly, a symmetrical quadrupole field is developedwithin the space enclosed by the electrodes.

The shape of this field is indicated by the equipotential lines 55 ofFigure 3c. The circular path within the field of the quadrupolerepresents the orbit of the electrons which is coursed at the cyclotronfrequency and the four arrows along the axes show the forces exertedupon the electrons in the four regions or quadrants of the structure.The electron represented by the filled smaller circle at the top left ofthe electron orbital path encounters the forces indicated whichaccelerate it along its clockwise path. Another electron, shown as anempty circle at the top right of the electron path, is subjected toforces which decelerate its orbital motion. It is to be noted that thereis no field at all at the center of the quadrupole and that the fieldintensity increases linearly with distance from the center. As aconsequence, the forces exerted upon an orbiting electron areproportional to the radius of the circle in which it moves so that theradius must increase or decrease exponentially.

This amplification mechanism may be viewed by resolving the alternatingquadrupole field pattern into two counter-rotating component patterns inthe manner commonly employed in analyzing rotating electrical machines.Because the structure at hand has four poles, the component patternscomplete one revolution for every two cycles of the pump frequency. Thusone of the patterns revolves synchronously with the orbiting electrons;the other one may be neglected. An electron which enters in the bestphase condition with respect to the synchronous pattern remains in thatphase throughout its passage through the quadrupole and it may also beshown that a phase-focusing process exists whereby an electron whichenters with an intermediate phase is shifted toward the best phaseposition.

While the field pattern within the modulation expander of the presentinvention, featuring a helical transmission line disposed in a helicalpattern about the beam path, is not identically the same as that of thequadrupole structure of Figure 30, they are indeed very similar andaccomplish amplification in the same way.

The field of a quadrupole having electrodes of hyperbolic configurationis described by the equation:

V=V (y x where, V is proportional to the instantaneous potential appliedto quadrupole electrodes and, x and y are Cartesian coordinates.Equation 2 may be transferred to polar coordinates by substituting theusual conversion factors for x and y as follows:

x=r cos 6 (3) y=r sin 0 (4) Accordingly, Equation 2 may be rewritten asfollows:

and by means of a trigonometric identity, Equation 5 converts into:

For a circle of constant radius r the potential is thus seen to be acosine function of the azimuth 0, experiencing two complete cycles inone traverse around the circle. This is precisely the potentialdistribution along the circumference of the helical modulation expanderof the present invention in a case where the pump frequency is twice thecyclotron frequency.

A significant difference does, however, exist between the quadrupoletype of modulation expander and the helical modulation expander ofFigure 2. As the pump wave travels around the circumference of thehelical expander, the quadrupole field which it establishes within theexpander rotates in a single direction, namely, the direction of wavetravel around the helix. On the other hand, the application of analternating voltage to the quadrupole structure of Figure 3b establishesa field which can be represented by two counter-rotating componentpatterns. As explained above, only one of these component patterns isutilized for amplification. With the present invention, the sense ofpattern rotation is chosen to be consistent with the direction of thecyclotron magnetic field and no counter-rotating pattern is generated.

The condition of Figure 3a wherein the modulation expander of Figure 2is the analogue of the quadrupole expander of Figure 3b results from theuse of a pumping frequency which is twice the cyclotron frequency butthere may be occasions when it is highly desirable to use a differentpumping frequency. As explained earlier,

this may be the case where one chooses to shift the frequency of theidler wave component which results as a modulation product in theprocess of expanding the modulation of the beam. If the selected pumpingfrequency is greater than twice the cyclotron frequency, the

instantaneous potential distribution along helix 40 of the' modulationexpander at a given instant may be that represented in Figure 3d ratherthan that of Figure 3a. The result is an effective rotation of thequadrupole field within the expander as seen by electrons movingtherethrough. This, of course, follows since at pump frequencies greaterthan twice the cyclotron frequency, the effective electrical length ofeach turn of the helix exceeds twice the wave length of the pumpgenerator. This is analogous to a skewed quadrupole as represented inFigure 32 which is an expander structure described and claimed in theaforesaid Wade application. Its electrodes 153 are conductive stripsskewed in a direction opposed to the direction of orbital movement ofthe electrons. Alternatively, the pumping signal frequency may be lessthan twice the cyclotron frequency which again results in effectiverotation of the quadrupole field within the helix of the expander butthis time in a direction opposite to that of Figure 32, that is, skewedin the direction of orbital movement of the electrons. In either casethe operation of the device in accomplishing expansion of the 7 beammotion is generally similar to that described in conjunction withFigures 3a3c.

The axial propagation of the pump wave must be properly related to thevelocity of electrons along the axial path 11, 11. For the particularcase in which the pump frequency is precisely that required to generatetwo cycles along the circumference of the helix, no specific relationbetween these two velocities is required. However, to accommodateoperating conditions in which the pump frequency deviates from thatnominal value, it is advantageous to design the helix, by appropriateselection of its parameters, to the end that the axial component of thepropagation velocity corresponds with the velocity of the stream. Ifthis is done and if the nominal pump frequency coincides with twice thecyclotron frequency, every electron keeps pace with a specific portionof the pump wave traveling about the circumference of the expanderhelix; this remains true regardless of the pump frequency, permittingthe expander helix to be made as long as desired without limiting therange of usable pump frequencies.

Arrangements of the type represented in Figure l employing input andoutput couplers that use deflectors as a slow wave circuit forinteracting with the beam are particularly suited to a condition of fastwaves of infinite phase velocity on the beam; that is a condition inwhich the cyclotron frequency is the same as the signal frequency. Insome installations it may be desirable to operate with a cyclotronfrequency distinctly different from the signal frequency, being eitherhigher or lower. This will result in fast waves of finite phase velocitydeveloped on the beam in the input modulator but of the forwardly orbackwardly directed type as explained in the Adler application, whereforward is the direction from source 10 to anode 12 and backward is theopposite direction. For such finite phase conditions, more expeditiousmodulation of the beam with signal energy and stripping of undesiredsignal components carried by the beam into the input modulator isaccomplished by the use of transmission line couplers. Modulationexpansion and signal gain may be achieved with the arrangement of Figure2 for either condition. In other words, the arrangement described isuseful in diiferent combinations of finite and infinite phase velocityphenomenon in the three sections constituted by the couplers and themodulation expander. Where different conditions of phase velocity aredesired, it may be more appropriate to employ separate solenoids atthese portions of the beam path in order to separately develop thefocusing fields.

The described arrangement introduces a very desirable flexibility intothe operation of the parametric amplifier and does so by means of asimplified structure. ti'cular it accommodates operation with a pumpingfrequency selected within a band of frequencies so that the Inparrelative frequencies of the signal and idler wave components may beadjusted in accordance with the requirements of individualinstallations.

While a particular embodiment of the invention has been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

I claim:

1. In an amplifying system in which an electron beam modulated withsignal energy is projected along a predetermined path, a parametricmodulation expander for operation over a band of pumping frequenciescentered about a given frequency comprising: a transmission line wrappedabout a portion of said path in accordance with a generally helicalpattern and having an electrical length per turn which is equal to twicethe Wave length at said given frequency.

2. In an amplifying system in which an electron beam modulated withsignal energy is projected along a predetermined path, a parametricmodulation expander for operation over a band of pumping frequenciescentered about a given frequency comprising: a transmission line wrappedabout a portion of said path in accordance with a generally helicalpattern, having a substantially constant velocity of propagation oversaid band, and having an electrical length per turn which is equal totwice the wave length at said given frequency.

3. In an amplifying system in which an electron beam modulated withsignal energy is projected along a predetermined path, a parametricmodulation expander for operation over a band of pumping frequenciescentered about a given frequency comprising: a helical transmis sionline wrapped about a portion of said path in accordance with a generallyhelical pattern, having a substantially constant velocity of propagationover said band, and having an electrical length per turn of said patternwhich is equal to twice the wave length at said given frequency.

4. In a parametric type of amplifying system having a predeterminedelectron resonance frequency and in which an electron beam modulatedwith signal energy is projected along a predetermined path with aparticular velocity, a parametric modulation expander for operation overa band of pumping frequencies comprising: a transmission line Wrappedabout a portion of said path in accordance with a generally helicalpattern, having an electrical length per turn which is equal to twicethe wave length at a pumping frequency corresponding to twice saidresonance frequency and having an axial component of propagationvelocity substantially equal to the velocity of said beam.

No references cited.

